1. Field of the Invention
The invention relates to a measurement method for measuring the birefringence of an optical measurement object, and to a measurement system suitable for carrying out the measurement method.
2. Description of the Related Prior Art
In many gases, liquids and stress-free amorphous solids, for example optical glasses, the speed of light is independent of the direction of propagation and of the polarization state of the light. Such optical media are referred to as optically isotropic. By contrast, if the optical properties of a material are dependent on the direction of propagation of the light, then the material is referred to as optically anisotropic. Many transparent crystalline materials are optically anisotropic. On account of the symmetry of their crystal lattice, they have at least one distinguished direction of symmetry, which is generally referred to as “optical crystal axis”.
Many optically anisotropic materials exhibit birefringence. The term birefringence designates the property of optically anisotropic materials to split an incident light beam into two partial beams which are linearly polarized perpendicularly to one another and which propagate in different ways in the optically anisotropic material. The different propagation of light in anisotropic materials is substantially determined by the dependence of the speed of light on the direction of propagation and on the polarization state of the light. The propagation speed of one of the partial beams is independent of the direction of propagation. This partial beam is referred to as the “ordinary ray”. By contrast, the propagation speed of the other partial beam is direction-dependent. This partial beam is referred to as the extraordinary ray. Associated with the different propagation speeds are correspondingly different refractive indices of the material for the different partial beams, where no is the refractive index for the ordinary ray and neo is the refractive index for the extraordinary ray. The birefringence based on the crystal structure of optical materials is referred to as intrinsic birefringence.
Optically isotropic materials can become birefringent as a result of external influences. Thus, by way of example, the birefringence induced by electric fields is used in the Kerr effect. In the case of intrinsically birefringent materials, the birefringent properties can change on account of external influences. In particular, mechanical stresses can induce birefringence, which is generally referred to as stress birefringence. Stress birefringence can be induced by internal stresses that result for example from the process for producing a crystal material. Furthermore, stress birefringence can be generated by external forces that arise e.g. in the course of mounting optical components in the mounts.
The birefringence is used as a desired property in the production of retardation elements (retarders), such as λ/4 plates or λ/2 plates, for example, or in the production of other polarization-optical components, in order to alter the polarization state of light in a defined manner.
On the other hand, in many demanding applications, for example in the field of microlithography, laser optics or astronomy, the birefringence of optical components is treated as an undesirable cause of error and endeavours are made to minimize the influence of birefringence on the optical properties of optical components or optical systems and/or to know it at least precisely enough that compensation is possible.
A precise knowledge of the extent of the birefringence, both in terms of the absolute value and in terms of the orientation of the birefringence, is important for controlling the birefringence. Therefore, there is a desire in the art for precise measurement methods for quantifying the birefringence.
Particularly stringent requirements made of the measurement accuracy and the capability of precisely determining even relatively weak birefringent effects exist in the field of optical systems for microlithography, which is used in particular in the production of large scale integrated semiconductor components and other finely structured components. In order to be able to produce ever finer structures with the aid of microlithography, the image-side numerical apertures of projection objectives are being increased ever further and ever shorter wavelengths are being used, in particular from the deep ultraviolet range (DUV). At wavelengths of less than 200 nm, only relatively few sufficiently transparent materials are available for producing transparent optical elements. They include primarily synthetic fused silica, which is sufficiently transparent down to 193 nm, and also some fluoride crystal materials, such as e.g. calcium fluoride or barium fluoride, which still exhibit sufficiently low absorption even at wavelengths of 157 nm and below. Calcium fluoride exhibits an intrinsic birefringence, i.e. a birefringence attributable to the crystal structure of the material, which, in addition to a possibly induced stress birefringence, can influence the polarization-optical behaviour of optical components composed of this material (cf. e.g. U.S. Pat. No. 6,697,199 B2 and literature citations indicated therein).
Each individual optical component exhibiting birefringence can make complex contributions to the polarization-optical behaviour of a system. Particularly in the field of microlithography, use is made of complex optical systems having a multiplicity of individual components which are often combined to form optical modules which perform specific functions within an overall optical system. In this case, it is generally desirable to know precisely both the birefringent properties of the overall system and the contributions of individual components or modules to the polarization-optical behaviour of the overall system.
In order to quantify the birefringence, measurement methods and measurement systems for measuring the birefringence of optical measurement objects are used, where the optical measurement object can be an individual optical component or a system comprising a plurality of optical components.
In the case of the measurement methods and measurement systems for quantifying the birefringence which are under consideration here, a measurement beam having a defined input polarization state is generated, said measurement beam being directed onto a measurement object, the input polarization state being the polarization state of the measurement beam directly before the measurement beam enters into the measurement object. After interaction of the measurement beam with the measurement object, polarization properties of the measurement beam are detected in order to generate polarization measurement values representing an output polarization state of the measurement beam, the output polarization state being the polarization state of the measurement beam after interaction with the measurement object.
The polarization measurement values are evaluated in order to determine at least one birefringence parameter representing the birefringence of the measurement object. In general, the absolute value and the orientation of the birefringence are determined. In this case, the absolute value of the birefringence represents the retardation—caused by the measurement object—between the two partial beams of the measurement beam which propagate at different propagation speeds in the material. The retardation between the two partial beams, which is also referred to as the optical path difference, is usually specified in nanometers or in fractions of the wavelength λ of the measurement beam. Thus, by way of example, a λ/4 retarder at a measurement wavelength of 193 nm generates a path difference of 193/4 nm.
The orientation of the birefringence is defined by the orientation of the optical crystal axis of the birefringent material. If optically isotropic materials are involved which become birefringent as a result of an external influence, such as e.g. force action, then the orientation of the birefringence lies in the direction of the acting force. For the purposes of a measurement, the orientation of the birefringence can be expressed by angle indications relative to a defined reference direction of the measurement system.
A precise measurement presupposes that the input polarization state is set as precisely as possible and that the output polarization state is determined as precisely as possible. Errors arising when the input polarization state is generated and when the output polarization state is determined influence the measurement as measurement errors. These measurement error contributions should therefore be known or determinable in order to be able to be taken into account in the evaluation.
If, by way of example, the measurement is effected with the aid of a polarimeter or ellipsometer according to the Sérnarmont principle, then firstly a linearly polarized measurement beam is generated from the light from an unpolarized light source with the aid of a polarizer, said measurement beam entering into the measurement object. A birefringence within the measurement object generally leads to an elliptically polarized output polarization state. With the aid of a quarter-wave plate, linearly polarized light is generated again from the elliptically polarized light and its polarization angle can be determined with the aid of a rotatable analyzer arranged upstream of a light-sensitive detector.
U.S. Pat. No. 6,697,157 B2 and U.S. Pat. No. 6,473,181 B1 describe systems for measuring birefringence wherein a photoelastic modulator (PEM) is used for modulating polarized light, which is then radiated through a sample to be measured.